The present invention generally relates to drop-out detection circuits, and more particularly to a drop-out detection circuit which detects a missing part of a record signal.
Conventionally, a storage medium like a magneto-optic disk on which a prescribed data is stored sometimes has a flaw or foreign matter on a surface thereof. In such a case, a record signal reproduced from the storage medium shows a voltage level below a prescribed level instantaneously at a portion of the storage medium where the flaw or foreign matter exists, and a part of the recorded data corresponding to that portion of the storage medium is lost. This kind of problem which arises primarily when the recorded data is produced is called hereinafter a drop-out. Once this drop-out is accurately detected within the record signal reproduced from the storage medium, it is possible to substitute a pseudo-signal for the missing part of the record signal corresponding to the drop-out detected to maintain the continuity of the recorded data.
FIG. 1 shows an example of a conventional drop-out detection circuit. In FIG. 1, a modulated signal such as a frequency modulated signal (FM signal) is supplied to an input terminal 20 of this conventional drop-out detection circuit. This modulated signal has a waveform as shown in FIG. 2A, and a voltage of the modulated signal is compared with a reference voltage by a comparator 21 to supply a rectangular wave signal as indicated in FIG. 2B. As shown in FIG. 2B, the rectangular wave signal changes from a low level to a high level when the modulated signal crosses a zero level from a negative region to a positive region, and the rectangular wave signal returns from the high level back to the low level when the modulated signal crosses the zero level from the positive region to the negative region. This rectangular wave signal is supplied from the comparator 21 to a retrigger monostable multivibrator (RMM) 22, and this RMM 22 outputs a pulse signal, as shown in FIG. 2C, to an output terminal 24. This pulse signal is supplied through overlapping of pulses which are generated from the RMM 22 at rise points (where a zero crossing from negative level to positive level occurs) of the above rectangular wave signal supplied from the comparator 21. As can be seen from FIG. 2C, the pulse signal outputted from the RMM 22 is maintained in a metastable state (high level) when a pulse is supplied from the comparator 21 to the RMM 22 within a predetermined time period that is based on a time duration T of a pulse signal outputted from the RMM 22, and this time duration of the pulse signal is set up with a variable resistor 23 that is connected to a terminal of the RMM 22. However, the above pulse signal outputted from the RMM 22 returns to a low level when a pulse within the rectangular wave signal is not supplied to the RMM 22 within the prescribed pulse duration T. When the modulated signal outputted to the comparator 21 has a drop-out which may cause the loss of a few pulses within the modulated signal as indicated by a dotted line in FIG. 2A, the comparator 21 does not supply a pulse of the rectangular wave signal corresponding to the lost pulses of the modulated signal to the RMM 22 within the prescribed pulse duration T, and a time longer than the prescribed pulse duration elapses until the following pulse of the rectangular wave signal is supplied from the comparator 21 to the RMM 22. Then, a negative pulse of the pulse signal, as shown in FIG. 2C, is supplied from the RMM 22 to the output terminal 24, and this negative pulse constitutes a major part of the drop-out detection signal that is supplied by the conventional drop-out detection circuit.
When this conventional drop-out detection circuit is used, it is necessary that the pulse duration T of the pulse signal supplied from the RMM 22 be represented by the formula a&lt;T&lt;d (where "a" denotes one period of the modulated signal, and "d" denotes a time period of the lost part of the modulated signal). With the pulse duration T satisfying the formula above, it is possible to detect any lost pulse within the inputted modulated signal due to the drop-out for one pulse duration of the modulated signal. If the variable resistor 23 is adjusted to have a resistance which allows the formula above to be satisfied, the conventional drop-out detection circuit can detect a lost part of the modulated signal as the result of the pulse signal with the low level being outputted as a drop-out detection signal from the RMM 22 to the output terminal 24.
However, in the conventional drop-out detection circuit, when a temperature around the circuit changes greatly, the pulse duration T of the pulse signal outputted from the RMM 22 varies significantly. And, the frequency of the frequency modulated signal inputted to the input terminal 20 often fluctuates. This fluctuation of the frequency of the FM signal is, for example, about 25% in a range between 5.2 MHz and 6.7 MHz and about 20% in a range between 8 MHz and 10 MHz. Therefore, in the case of the conventional drop-out detection signal suffering a change in ambient temperature or a fluctuation of the frequency, it is difficult to preset an appropriate pulse duration T of the pulse signal outputted from the retrigger monostable multivibrator which is represented by the formula: a&lt;T&lt;d, as described above.